The Electric-Field Gradient (EFG)
at 111Cd impurity site in a-Al2O3
has been extensively studied by means of the TDPAC technique (using the 111In/111Cd
and 111mCd /111Cd ion-implanted PAC probes) and,
due to the excellent quality of the experimental spectra and to the presence of
a very well-defined quadrupole frequency with a very small distribution, the
authors assigned the hyperfine interaction as generated by 111Cd
probes located at the substitutional Al site [1,2]. Indeed, a single
well-defined hyperfine interaction just only proves that the tracers are all
occupying the same crystalline site, either substitutional or interstitial. On
the other hand, Rutherford Backscattering/Channeling spectroscopy (RBS/C)
experiments have shown that In impurities implanted in a-Al2O3
occupy interstitial sites [3]. In order to elucidate this controversy we
performed a DFT study obtaining the EFG at Cd impurities located at cationic
and interstitial sites in the semiconductor a-Al2O3
using two state-of-the-art ab initio all-electron
methods: the Full-Potential Augmented Plane Wave plus local orbitals
(FP-APW+lo) and the Projector Augmented Wave (PAW). For this study we obtained
the same EFG for substitutional Cd in a negatively ionized charge state and for
Cd at interstitial sites in a neutral charge state, and therefore we could not
decide the impurity site localization.
In this work we present a
detailed study of formation energies of Cd impurities in a-Al2O3
at both substitutional and interstitial sites and for different charge state of
the impurity. We found that for each scenario (substitutional or interstitial) the
lower energy corresponds with the charge state that reproduces the experimental
EFG. Finally, we could resolve this indetermination showing that 111Cd
impurities occupying the substitutional Al site in the a-Al2O3
host, in the negative ionized charge state, have the lowest formation energy, and
should be the correct scenario.